polymers Article Plasticized Polystyrene by Addition of -Diene Based Molecules for Defect-Less CVD Graphene Transfer Tuqeer Nasir 1 , Bum Jun Kim 1, Muhammad Hassnain 2, Sang Hoon Lee 2, Byung Joo Jeong 2, Ik Jun Choi 2, Youngho Kim 3, Hak Ki Yu 3,* and Jae-Young Choi 1,2,* 1 SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 16419, Korea; [email protected] (T.N.); [email protected] (B.J.K.) 2 School of Advanced Materials Science & Engineering, Sungkyunkwan University, Suwon 16419, Korea; [email protected] (M.H.); [email protected] (S.H.L.); [email protected] (B.J.J.); [email protected] (I.J.C.) 3 Department of Materials Science and Engineering & Department of Energy Systems Research, Ajou University, Suwon 16499, Korea; [email protected] * Correspondence: [email protected] (H.K.Y.); [email protected] (J.-Y.C.) Received: 19 June 2020; Accepted: 14 August 2020; Published: 17 August 2020 Abstract: Chemical vapor deposition of graphene on transition metals is the most favored method to get large scale homogenous graphene films to date. However, this method involves a very critical step of transferring as grown graphene to desired substrates. A sacrificial polymer film is used to provide mechanical and structural support to graphene, as it is detached from underlying metal substrate, but, the residue and cracks of the polymer film after the transfer process affects the properties of the graphene. Herein, a simple mixture of polystyrene and low weight plasticizing molecules is reported as a suitable candidate to be used as polymer support layer for transfer of graphene synthesized by chemical vapor deposition (CVD). This combination primarily improves the flexibility of the polystyrene to prevent cracking during the transfer process. In addition, the polymer removal solvent can easily penetrate between the softener molecules, so that the polymer film can be easily dissolved after transfer of graphene, thereby leaving no residue. This facile method can be used freely for the large-scale transfer of 2D materials. Keywords: graphene transfer; polymer plasticization; flexible polymer film; polystyrene; polymer functionalization 1. Introduction Since the realization of graphene flakes [1], a lot of research has been carried out due to its exceptional electrical and physical properties [2,3]. Several top down and bottom up approaches have been explored for high quality graphene synthesis by various research groups [4–6]. Among these technologies, the chemical vapor deposition (CVD) method has caught the attention of researchers due to its advantages in synthesizing large scale and high-quality graphene, using a transition metal substrate as catalyst [7–13]. This method, however, required an additional process of transferring the synthesized graphene from metal substrate to target substrates like SiO2 for device fabrications [14–16]. Among several transfer techniques, poly (methyl methacrylate) (PMMA) assisted graphene transfer method [17] has been adopted widely, due to simplicity process and strong adhesion between graphene and PMMA [18]. However, the strong adhesion can cause permanent bonding between PMMA and graphene layer, due to the presence of active bonding sites in PMMA molecule chain like hydroxyl and carboxyl links, which can attach to graphene edges and cannot be removed during subsequent dissolution step of removing this polymer layer after transfer [19–23]. The other main issue regarding Polymers 2020, 12, 1839; doi:10.3390/polym12081839 www.mdpi.com/journal/polymers Polymers 2020, 12, 1839 2 of 10 Polymers 2020, 12, x FOR PEER REVIEW 2 of 10 PMMA assisted graphene transfer is related to intrinsic mechanical properties of the support layer ofpolymer the support itself, layer since polymer it must itself, provide since enough it must mechanical provide enough support mechanical and flexibility support to grapheneand flexibility layer toduring graphene its transfer. layer during Otherwise, its transfer. it can Otherwise, cause mechanical it can cause defects mechanical like cracks defects and like holes cracks in transferred and holes ingraphene, transferred due tographene, poor flexibility due to and poor hardness flexibilit valuesy and of polymerhardness support values layer of polymer (PMMA) support itself. Recently, layer (PMMA)a number itself. of new Recently, materials a number have been of new explored materials to avoidhave been the residue explored forming to avoid issue the foundresidue in forming PMMA issueassisted found graphene in PMMA transfer, assisted which graphene range transfer, from using which water range soluble from polymersusing water to soluble paraffin polymers [24–27]. to To paraffinthe best [24–27]. of our knowledge, To the best of mechanical our knowledge, properties mechanical of these properties support layer of these materials support and layer their materials effect on andquality their transferred effect on quality graphene transferred have not graphene been studied have before. not been studied before. ToTo remove remove the the complications complications regarding regarding the the wet wet transfer transfer of of graphene graphene altogether, altogether, recently, recently, we we reportedreported the the use use of of polystyrene polystyrene assi assistedsted graphene graphene transfer transfer with with the the addition addition of ofplasticizer plasticizer named named 4, 44,′-Di-iso-propylbiphenyl 40-Di-iso-propylbiphenyl (DIPB) (DIPB) [28]. [28 Polystyrene]. Polystyrene was was used used to toprovide provide stable stable and and residue residue free free adhesionadhesion with with underlying underlying graphene, graphene, and and DIPB DIPB is is added added to to improve improve poor poor mechanical mechanical and and physical physical propertiesproperties of of polystyrene polystyrene itself. itself. Resultant Resultant polymer polymer support support film film demonstrated demonstrated excellent excellent properties properties of of transferredtransferred graphene graphene [28]. [28]. The The large large scale scale transfer transfer of of graphene graphene using using plasticized plasticized polystyrene polystyrene showed showed viableviable method method of of roll roll to to roll roll transfer transfer of of as-grown as-grown CVD CVD graphene. graphene. InIn this this study, study, the the effects effects of of the the length length of of plasticizer plasticizer molecules molecules mixed mixed with with polystyrene polystyrene on on the the mechanicalmechanical properties properties and and flexibility flexibility were were systematically systematically tested tested and and analyzed. It It is is intended intended to to furtherfurther develop develop previous previous published published studies studies by by applying applying to to graphene graphene transfer transfer experiments. experiments. We We have have usedused polystyrene polystyrene as as base base polymer polymer for for graphene graphene tran transfer,sfer, due due to to the the presence presence of of multiple multiple aromatic aromatic ringsrings in in polystyrene polystyrene polymer polymer ch chain,ain, which which can can form form strong strong ππ--ππ linkslinks with with graphene graphene layer layer for for strong strong adhesion.adhesion. Since Since polystyrene polystyrene does does not not contain contain any any acti activeve bonding bonding links links in in its its molecular molecular structure, structure, any any unwantedunwanted and and permanent permanent bonding bonding between between the the polyme polymerr and and graphene graphene can can be be avoided. avoided. Furthermore, Furthermore, polystyrenepolystyrene is is readily readily soluble soluble in in wide wide array array of of organic organic solvents, solvents, leaving leaving no no residues residues after after dissolution. dissolution. PolystyrenePolystyrene has has intrinsically intrinsically amorphous amorphous structure structure and and mechanical mechanical properties properties of of polystyrene polystyrene itself itself are are notnot very very suitable suitable for for graphene graphene transfer, transfer, due due to to its its rigid rigid structure structure [29,30]. [29,30]. So, So, we we introduced introduced low low weight weight plasticizerplasticizer molecules molecules during during solution solution making making of of po polystyrenelystyrene to to improve improve the the mechanical mechanical flexibility flexibility of of thethe support support layer layer film film when when used used for for graphene graphene tr transfer.ansfer. Furthermore, Furthermore, 1-5 1-5 hexadiene, hexadiene, 1-7 1-7 octadiene octadiene andand 1-9 1-9 decadiene decadiene molecules (similar molecular structurestructure butbut hashas di differentfferent length, length, See See the the Figure Figure1a 1a of ofconcentration concentration of upof toup 20 to w.t% 20 werew.t% used were for used the plasticization for the plasticization of polystyrene. of polystyrene. This simple plasticizationThis simple plasticizationprocess helps process to provide helps the to perfect provide combination the perfect of combination properties desired of properties for an e ffdesiredective polymerfor an effective layer to polymerbe used inlayer graphene to be transfer,used in withoutgraphene any transfer, unwanted without side e ffanyects unwanted of residues side or mechanical effects of residues defects. Theor mechanicalresults reported defects. in thisThe workresults demonstrate reported in thethis possibility work demonstrate of this new